The air duct system is the most important part of air conditioning and ventilation engineering. Its main function is to transfer the regulated air to the terminal equipment as efficiently as possible according to the design flow. Generally, there are three shapes of duct cross section: rectangular, circular and oblate. Rectangular air duct is usually made by riveting four steel plates. The circular air duct is made of 137 mm wide steel plate on the circular air duct machine. Flat circular ducts are rare. It is generally formed by extruding the round duct through the round duct machine. Before 1960, because of the simple manufacturing process and the small installation space, most of the ventilation systems used rectangular ducts. With the successful development of large-scale spiral circular air duct machine, a large number of engineering examples have proved that circular air duct is far better than rectangular air duct in economy or other engineering parameters.
At present, the widely used fabric air duct in the market is an air distribution system which integrates the functions of tuyere, air supply pipe, static pressure box, thermal insulation material, air valve, etc. It has the advantages of accurate and even air supply, light weight installation block, high aesthetics, anti-bacterial and anti mildew, etc., which are recognized by users and widely used. The shape of the air duct made of fiber fabric has circle, semicircle, 1 / 4 circle, ellipse and semi ellipse to meet the needs of different building structures.